Dialysis apparatus is used as an artificial kidney for dialyzing blood, as well as for many other purposes such as purification of various chemical or biological products, and the like.
Solutes in the various solutions which are brought into contact with a dialysis membrane pass through the membrane at a rate which is related to the difference in the concentration of such solutes, at opposite sides of the membrane. However, water tends to pass through dialysis membranes in a manner which is more dependent upon the transmembrane pressure, and is less dependent upon concentration. Accordingly, during blood dialysis, for example, the amount of water which passes from the blood of the patient across the dialysis membrane (which phenomenon is known as "ultrafiltration") is dependent upon the overall transmembrane pressure, which in turn is dependent upon the pressure drop in the blood and dialysis solution flow paths of the dialyzer, and other pressure factors.
The amount of ultrafiltration from the blood which takes place is, of course, a critical medical factor in the treatment of a uremic patient. Physicians who perform dialysis upon their patients generally desire that a specific, pre-determined amount of ultra-filtration take place during each dialysis operation. Conventionally, the patient may be weighed, and his weight loss calculated in order to determine the amount of ultrafiltration which has taken place. However, this rather cumbersome method does not account for other factors which may cause the weight to vary, such as eating or elimination during the four to six hour course of blood dialysis, and loss of weight through perspiration and the like.
Accordingly, there is a need for a technique for directly measuring the amount of ultrafiltration, while at the same time permitting a large amount of dialysis solution to pass through the dialyzer, for example by means of the very popular "single pass" mode of dialysis, in which the dialysis solution passes only once through a stacked plate type or fiber type dialyzer and then is discarded. The same need exists in the "recirculating-single pass" technique of dialysis in which large volumes of dialysis solution are passed through a coil type dialyzer in a semi-recirculating mode, with dialysis solution being continuously added and withdrawn from the system.
The REDY dialysis system, which is commercially available at the present time, can utilize the direct measurement of ultrafiltration by measuring any increase in the total volume of dialysis solution used. This, however, is possible only because a relatively small amount of dialysis solution, measuring only a few liters, is used. The dialysis solution is continuously recirculated through the dialyzer, being continuously purified. This technique has not been chosen by the majority of doctors, who appear to prefer the use of a large volume of dialysis solution, which precludes the accurate measurement of total volume of dialysis solution as a means for measuring the amount of ultrafiltration.
In accordance with this invention, a system is provided which permits the direct measurement of ultrafiltration in a system in which dialysis solution may be added and withdrawn on a continuous basis, or on any other basis as desired. Accordingly, the currently preferred medical dialysis techniques can be performed, while at the same time one can directly determine the total amount of ultrafiltration which has taken place at the time during the dialysis operation. Accordingly, the physician can adjust the conditions of the dialysis operation to increase or decrease the rate of ultrafiltration as his judgment would indicate.